1. Field of the Invention
The present invention is related to a method and apparatus for locationing a instrument of tablet, especially to a method and apparatus in which a location is searched by a tablet instrument.
2. Description of the Prior Art
In the prior art, such as that discloses in the U.S. Pat. Nos. 4,210,775 and 4,423,286 and the U.S. Pat. No. 4,428,192 which is disclosed by the inventor of the present invention, in general, a coil is arranged in the front end of pen, and meanwhile an alternating current signal with fixed frequency and fixed amplitude is applied to the coil, therefore, a sense line grid which comprises some spaced conductors on the tablet may induce a relative alternating current signal.
For example, in U.S. Pat. No. 4,210,775, a scanning circuit is used to scan said spaced conductors, firstly it scans along X direction and then the Y direction. For each directions, the scanning process begins from the first conductor. The sequentially induced signal is input to a signal amplifier, each time the scanning process begins a counter is reset and start to count. The count operation is continued while the scanning process is going on, and the counter is increased in a fixed frequency higher than the scanning rate.
The output of said amplifying circuit is connected to the input of a synchronous phase detector, when the scanning process is scanning across the pen tip i.e. "from one side of the pen tip to another side thereof", the polarity of the output signal of the synchronous phase detector will be change. Then the signal is passed through a low pass filter to a comparing device. A pulse signal with rising edge is produced at the "Zero-Crossing" point of the filtered signal. The said pulse signal is used to stop the said count operation of the counter. The final value in the counter is then recorded, which represents the location of the pen tip in a sense line grid.
The aforementioned technology has been improved by the inventor of the present invention, and the improved result is disclosed in the U.S. Pat. No. 5,428,192, in which the start point of the count operation is changed. The said zero-crossing point of the filtered signal is used as a trigger to start a count operation, as the scanning is switched to the following next sense line the count operation is stopped. Therefore, the counting period is reduced and the length of the counter is also greatly reduced.
As described in U.S. Pat. No. 4,423,286, during the operation of sequentially scanning the induced signal from each of the spaced conductors is passed through a rectifier, a filter, and a Analog-Digital converter to obtain the amplitude of induced signal thereof, wherein the polarity of the phase angle of each said induced signal is detected by phase detector. When the polarity of the signal is varied during scanning, the amplitude of positive and negative signals may be substituted into a formula for operation, thus the location of the pen tip may be determined.
According to the aforementioned methods, the phase of said induced alternating signal has to be obtained to produce the zero-crossings during the process, as shown in FIG. 1, However, during the process of signal amplifying and filtering, if the process is affected by internal or external noises or by the tolerance of components so that there may be a DC offset voltage Voffset on the filtered signal, then the zero-crossing point will drift, a count error d will be generated.
Furthermore, the phase of the induced alternating current signal is important for determining the location of the pen, then the phase signal from the coil in the pen must be obtained so as to compare with said induced signal to get the phase difference. In the conventional tablet design, the phase signal of the coil is connected to the scanning circuit by a wire. But now in practical usage, a wireless pen is desired for a better feeling when making a sketch or writing a signature. Thus a low cost wireless pen is difficult to implement.
Thus it is apparent that the methods and apparatus for determining the location of the instrument on a tablet still have some defects.